This invention relates generally to the fields of high frequency electromagnetic interference shielding an A.C. power isolation. It is particularly directed to the shielding of high frequency shielded conductor systems, such as coaxial cables, from electromagnetic interference and the simultaneous isolation of such conductor systems from sources of A.C. power. The 75 ohm coaxial cable input to a television tuner is a prime example of one type of shielded conductor to which such shielding and isolation is directed.
Television receiver manufacturers are currently required by Underwriters Laboratories (U.L.) to doubly isolate exposed metal parts from the A.C. line which powers the receiver. For example, the 300 ohm twin lead terminals usually situated on the rear of the receiver's cabinet are required to be separately isolated. Such isolation is intended to doubly insulate a consumer from accidental shock which he might otherwise receive either from contact with the exposed terminals or with the metal "rabbit ear" antenna to which such terminals are sometimes connected.
Conventionally, television receivers also include an exposed connection for a 75 ohm coaxial cable input to the receiver's VHF tuner. No U.L. requirement presently exists providing for double isolation of the coaxial input, evidently because the technology has not been available to television manufacturers to enable them to provide such isolation while simultaneously affording acceptable television reception.
The problem which arises in connection with the 75 ohm coaxial input is that conventional techniques for isolating the coaxial input from the A.C. line tend to permit ambient high frequency electromagnetic interference signal to couple with the field within the cable, and thus to interfere with the desired signal propagating inside the coaxial cable.
For example, one prior approach utilizes conventional capacitors coupling the coaxial cable with the tuner input to A.C. isolate the cable from the tuner. While the isolation thus achieved is satisfactory, the field within the cable is inadequately shielded from electromagnetic interference.
A more recent isolation technique, described in copending application Ser. No. 184,720, filed Sept. 8, 1980, employs a feed-through or tubular type capacitor in the cable for A.C. isolation. The latter arrangement does provide the required degree of A.C. line isolation but, in fields of strong ambient electromagnetic interference, its shielding effect is less than perfectly satisfactory.
The shielding problems mentioned above may be particularly evident where the coaxial cable, connected to the 75 ohm input, carries a CATV signal. If the cable includes an A.C. isolator which is an inadequate electromagnetic interference shield, strong co-channel ambient broadcast fields will not be adequately shielded from the field within the coaxial cable and will produce strong co-channel interference.
For the reasons stated above, presently available A.C. isolators have not proven adequate where electromagnetic interference shielding is of importance.